At present, magnetic recording reproduction devices are designed to have higher recording density in order to achieve a large capacity with a small size. In the field of a hard disc drive, which is a typical magnetic recording reproduction device, a device having an areal recording density of more than 3 Gbits/in2 (4.65 Mbits/mm2) is already commercialized, and such a rapid progress in the technology can be observed that a device having a surface recording density of 10 Gbits/in2 (15.5 Mbits/mm2) is anticipated to come into practical use in a few years.
As the technical background for enabling such higher recording density, there are improvements in the performance of a magnetic recording medium and a head-disc interface as well as improvements in the linear recording density achieved by the appearance of a new signal processing mode such as a partial response.
Here, the partial response is a mode of intentionally providing a known intersymbol interference at the time of waveform equalization conducted for avoiding an intersymbol interference when the linear recording density is increased, and this mode is characterized in that the deterioration of a bit error rate can be prevented compared to a conventional peak detection or an integral detection.
In recent years, however, in addition to the appearance of such a signal processing mode, the main factor for improving the surface recording density is that the tendency toward an increase in the track density is significantly exceeding the tendency toward an increase in the linear recording density. This is due to the fact that a magneto-resistive type head, which has excellent reproduction output performance compared to a conventional inductive type magnetic head, has come into practical use. At present, due to the commercialization of the magneto-resistive device type head, a signal with a track width of not more than several μm can be reproduced at a high S/N ratio. On the other hand, along with a further improvement in the performance of the head in the years to come, a track pitch is expected to reach the submicron range in the near future.
For a magnetic head to scan such narrow tracks accurately and to reproduce signals at a high S/N ratio, the tracking servo technology of the magnetic head plays an important role. As for such tracking servo technology of present hard disc drives, for example, “Yamaguchi: High precision servo technology of a magnetic disc device, Journal of Japan Society of Applied Magnetics, Vol. 20, No. 3, p. 771 (1996)” discloses the content thereof in detail. According to this document, recording tracks are formed in circular manner on a hard disc. And within a revolution of the disc, that is, within an angle of 360 degrees, a single region called a wedge is repeatedly provided at a constant interval, where a servo signal for tracking, an address information signal and a reproduction clock signal etc. are recorded (hereinafter referred to as a “preformat recording area”). With the use thereof, the magnetic head reproduces these signals at a constant interval and identifies its own position, and thus the magnetic head can scan on the tracks accurately while, if necessary, correcting for displacement of the magnetic disc in the radial direction.
Furthermore, since the above-mentioned preformat information signals such as a servo signal for tracking, an address information signal and a reproduction clock signal serve as reference signals for the magnetic head to scan accurately on the tracks, track positioning is required to be performed correctly with precision. For example, according to the content disclosed by “Uematsu, et al.: Mechanical Servo, Present state and outlook of the HDI technology, Materials for the ninety-third Workshop of Japan Society of Applied Magnetics, 93-5, pp. 35 (1996)”, in a present hard disc drive, after a magnetic disc and a magnetic head are incorporated into the drive, by using a single-purpose servo track recording device called a servo track writer, the servo signal for tracking, the address information signal and the reproduction clock signal etc. are recorded by the intrinsic magnetic head incorporated inside the drive.
In this case, the preformat recording is performed by precisely controlling the position of the intrinsic magnetic head incorporated inside the drive by an external actuator equipped in the servo tracking recording device so as to achieve the track positioning with the necessary precision.
However, the conventional technology of performing the preformat recording by the intrinsic magnetic head incorporated inside the drive with the single-purpose servo track writer has the following problems.
First, a recording by a magnetic head basically is a linear recording achieved by the relative movement of a magnetic head and a magnetic recording medium, so that the above-mentioned method for performing the recording by precisely controlling the position of the magnetic head with the single-purpose servo track writer requires a large amount of time for the preformat recording. Furthermore, since the single-purpose servo track writer is quite expensive, the cost needed for the preformat recording becomes higher.
This problem becomes more critical as the track density of a magnetic recording reproduction device is improved. In addition to the increased number of tracks in the radial direction of the disc, a longer time is required for the preformat recording due to the following reason. That is, as the track density is improved, the positioning of the magnetic head is required with high precision, so that an angle interval, at which the preformat recording area for recording the information signals such as the servo signal for tracking is provided in one circle of the disc, needs to be reduced. Therefore, as the device has higher recording density, the amount of signals to be recorded in the disc by the preformat recording is increased, and a large amount of time is needed.
Furthermore, there is a tendency for a magnetic disc medium to become smaller in diameter, but the demand for large-diameter discs of 3.5 inches and 5 inches also is still great. As the recording area of the disc is larger, the amount of signals to be recorded by the preformat recording is increased. Also the cost performance of such a large-diameter disc is affected significantly by the time needed for the preformat recording.
Secondly, due to a spacing between the magnetic head and the magnetic recording medium, and due to a broadening of a recording magnetic field caused by the pole shape of the tip of the magnetic head, the magnetic transition lacks in sharpness in the track edge portion of the recorded preformat signals.
Since the recording by a magnetic head basically is a linear recording achieved by the relative movement of a magnetic head and a magnetic recording medium, a certain amount of spacing must be provided between the magnetic head and the magnetic recording medium in view of the performance of the interface between the magnetic head and the magnetic recording medium. Moreover, due to the construction of the present magnetic head provided with two elements for performing recording and reproduction separately, a pole width on the trailing edge side of a recording gap corresponds to a recording track width, and a pole width on the leading edge side is at least twice as large as the recording track width.
The two problems mentioned above are both factors causing the recording magnetic field to broaden in the recording track edge portion. As a result, there are problems such as lack of sharpness of the magnetic transition in the edge portion of the recording track where the preformat recording was performed or clear areas created on both sides of the track edges. In the present servo tracking technology, the position of a magnetic head is detected based on the amount of change in the reproduction output at the time when the magnetic head went off-track and scanned. Therefore, the magnetic head is not only required to have an excellent S/N ratio when scanning accurately on the tracks just as when data signals recorded between servo areas are reproduced, but also to have a steep change in the reproduction output amplitude at the time when the magnetic head goes off-track and scans, that is, sharp off-track characteristics. Accordingly, when the magnetic transition in the track edge portion where the preformat recording was performed lacks in sharpness, it is difficult to provide the accurate servo tracking technology for recording of submicron tracks in the years to come.
In order to solve the two problems in the preformat recording by the magnetic head as mentioned above, JP10(1998)-40544A discloses the technology of using a master information carrier including a base on which a pattern of ferromagnetic thin films corresponding to preformat information signals is formed, and after bringing the surface of the master information carrier into contact with the surface of a magnetic recording medium, magnetizing the pattern of the ferromagnetic thin films formed on the master information carrier so as to record a magnetized pattern corresponding to the pattern of the ferromagnetic thin films in the magnetic recording medium. According to this preformat recording technology, an excellent preformat recording can be performed efficiently without sacrificing other important performance such an S/N ratio of the recording medium, interface performance and so forth.
According to the content disclosed by the same publication, the pattern of the ferromagnetic thin films corresponding to the preformat information signals such as the servo signal for tracking, the address information signal and the reproduction clock signal can be formed on the surface of the master information carrier by using the conventional photolithography technique.
FIG. 8 shows an example of this pattern array of ferromagnetic thin films. 22 is an array of ferromagnetic thin films.
FIG. 9 is a partial cross-sectional view of a master disc for a magnetic transcription used for transcribing servo signals in a magnetic disc by the method shown in the same publication. 21 is a master disc base, and 22 is a ferromagnetic thin film. The ferromagnetic thin films 22 are buried partially in the master disc base 21. As for the ferromagnetic thin film 22, soft magnetic materials having a high saturation magnetic flux density such as cobalt and permalloy are used.
FIG. 10 is a partial perspective view showing the configuration of a conventional master disc provided with the array of the ferromagnetic thin films 22 as mentioned above. 25 is a land portion provided on the master disc, which is configured to be joined with the surface of a magnetic disc when the magnetic disc is contacted closely therewith. Moreover, an array pattern portion 24 of the ferromagnetic thin films is distributed on the surface of the land portion 25. 26 is a concave portion having a predetermined difference in level with the land portion 25.
FIG. 11 is a plan view of the same conventional master disc. The broken line shows an outer diameter of a magnetic disc 27, in which information is transcribed by joining the master disc 23 with the magnetic disc 27 in the opposed state. The concave portion 26 is expanding radially from the central portion of the master disc 23 in the form of plural grooves and is closed at a point inward of the outer diameter of the magnetic disc 27. On the other hand, the land portion 25 is expanding radially from the central portion toward the periphery of the master disc 23 and is linked together at a point inward of the outer diameter of the magnetic disc 27.
In this way, when the magnetic disc 27 is contacted closely with the master disc 23 at the time of transcription, the concave portion 26 forms a radial space that is closed at the peripheral end portion of the magnetic disc 27 and open at the internal circumferential end portion of the magnetic disc 27.
FIG. 12 to FIG. 15 are drawings for explaining the process of a magnetic transcription performed in the magnetic disc 27 using the above-mentioned master disc 23. In these drawings, 28 is a spindle for supporting the magnetic disc 27, and 29 is a magnet generating a transcription magnetic field.
The first stage of the magnetic transcription is, as shown in FIG. 12, to closely contact the magnet 29 with the magnetic disc 27 and to perform a rotational scanning in the circumferential direction of the magnetic disc 27. Through this operation, as shown by the arrows in FIG. 14, first magnetization 30 magnetized in one circumferential direction remains on the entire surface of the magnetic disc 27.
The second stage of the magnetic transcription is, as shown in FIG. 13, to overlap the master disc 23 on the magnetic disc 27 that has been magnetized in one direction. Next, air is exhausted from a vent hole of the spindle 28 so as to emit the air present between the master disc 23 and the magnetic disc 27. Here, the air contained in the space formed by the concave portion 26 of the master disc 23 and the magnetic disc 27 is emitted, and a negative pressure is developed in the concave portion 26, so that the master disc 23 and the magnetic disc 27 are joined together.
Next, as in the first stage, the magnet 29 is contacted closely with the master disc 23, and a rotational scanning is performed in the circumferential direction of the magnetic disc 27. At this time, the rotational scanning direction may be either the same direction as in the first stage or the opposite direction to the first stage, but the polarity of the magnet 29 should be opposite to the polarity thereof in the first stage. Accordingly, as shown in FIG. 15, in the portion facing the array pattern portion 24 of the ferromagnetic thin films in the master disc 23, a pattern magnetization area 31 magnetized according to the array is formed, and furthermore, as shown by the arrows, second magnetization 32 magnetized in one circumferential direction remains in the portion other than the portion facing the array pattern portion 24 of the ferromagnetic thin films in the master disc 23.
The quality of signals recorded in the magnetic disc 27 by such a magnetic transcription is determined by the distance between the ferromagnetic thin film 22 and the surface of the magnetic disc 27 at the time when a transcription magnetic field is applied. That is, the quality is determined by how well the master disc 23 and the magnetic disc 27 are joined together.
With reference to FIG. 16, the problems of conducting a magnetic transcription in the magnetic disc 27 by using the conventional master disc 23 described above will be explained. In FIG. 16, 39 is a vacuum pump for emitting the air present between the magnetic disc 27 and the master disc 23. In the conventional master disc 23, as shown in FIG. 11, the concave portion 26 extending radially from the central portion of the master disc 23 is closed in front of the peripheral end portion of the magnetic disc 27.
Here, in the area where the concave portion 26 is present, the negative pressure due to the vacuum pump 39 is applied to the space formed by the concave portion 26, and due to the difference with the atmospheric pressure, the power of joining the magnetic disc 27 and the master disc 23 together is generated, but there is no such space in a peripheral portion 40 because the concave portion 26 is not present. In other words, in the peripheral portion 40, the power of joining the magnetic disc 27 and the master disc-23 together is not generated In such a portion where the adhesive pressure is not effected, the distance between the ferromagnetic thin film 22 of the master disc 23 and the magnetic disc 27 is not reduced sufficiently. Thus, there was a problem that the probability of causing transcription signal failure is high when a transcription is conducted.
Furthermore, a magnetic head and a magnetic disc in a hard disc drive have a gap of several nanometers, so that the presence of a minute foreign material on the magnetic disc poses a problem. Therefore, the process of manufacturing a magnetic disc includes the step of inspecting for foreign materials on a magnetic disc. This inspection generally is conducted by the method shown in FIG. 17. In FIG. 17, 53 is a laser beam emitted on the surface of the magnetic disc 27; 54 is a regular reflection component of the laser beam 53 reflected on the magnetic disc 27; and 55 is a reflected scattered light scattered by a foreign material on the magnetic disc 27. Conventionally, the reflected scattered light 55 scattered by a foreign material is detected by a detector 56 for judging whether a foreign material is present on the magnetic disc 27 or not.
However, in an internal circumferential edge 58a and a peripheral edge 58b of the magnetic disc 27 shown in FIG. 18, the laser beam 53 is likely to reflect irregularly, and even if there is no foreign material, the reflected scattered light 55 enters the detector 56 and leads the detector 56 to misjudge that a foreign material is present. Therefore, conventionally, as shown in FIG. 18, a foreign material detection range 57 in fact is determined as an area outside a predetermined distance from the internal circumferential edge 58a (generally between 0.1 mm and 0.5 mm) and inside a predetermined distance from the peripheral edge 58b (generally between 0.1 mm and 0.5 mm).
On the other hand, in the manufacturing process of a magnetic disc, the internal circumferential edge 58a or the peripheral edge 58b are held for transportation, so that the probability that a foreign material will be attached is high. Nevertheless, since the internal circumferential edge 58a and the peripheral edge 58b are not included in the foreign material detection range as described above, even when a foreign material 59 is attached on the internal circumferential edge 58a or the peripheral edge 58b of the magnetic disc 27, the probability that this substrate will pass the inspection to be used for manufacturing a magnetic disc is extremely high. FIG. 19 shows the problem of conducting a transcription in the magnetic disc 27 on which such a foreign material is attached. As shown in FIG. 19, at the portions where the foreign material 59 is attached on the magnetic disc 27, the surface of the master disc 23 and the surface of the magnetic disc 27 cannot be joined together and are separated. In such portions, the magnetic field on the surface of the magnetic disc 27 is disturbed, and a magnetic transcription of the information according to the array of the ferromagnetic thin films in the master disc 23 is not performed correctly in the magnetic disc 27.
In other words, since the internal circumferential edge 58a and the peripheral edge 58b of the magnetic disc 27 are not included in the foreign material detection range 57, even when the foreign material is present in these edge areas, the magnetic disc 27 passed the foreign material inspection, so that transcription failure occurred frequently in the internal circumferential edge 58a and the peripheral edge 58b. 
Furthermore, also in the manufacturing process of the master disc 23, the peripheral end portion of the master disc is held often for transporting the master disc, so that the probability that a foreign material will be attached on the peripheral end portion of the master disc also was high.
In the conventional master disc 23, as shown in FIG. 11, the area larger than the outer diameter of the magnetic disc 27 is the land portion 25. Therefore, the foreign material attached on the end portion of the master disc 23 by handing the master disc 23 is likely to be transferred to the land portion 25, which is the area contacting the magnetic disc 27. In particular, such a thing happens with a mucilaginous foreign material. The foreign material transferred to the contacting area hinders the master disc 23 from joining with the magnetic disc 27, thus causing transcription signal failure to occur.
As described above, conventionally, there also was the problem that the foreign material attached on the master disc or the magnetic disc hindered the adhesion between the master disc and the magnetic disc, thereby causing transcription signal failure to occur.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a master disc having strong adhesion with a magnetic disc and to achieve an excellent magnetic transcription without causing any unevenness over the entire magnetic disc.